1. Field of the Invention
The present invention relates to a planar magnetic element such as a planar inductor or a planar transformer.
2. Description of the Related Art
In recent years, electronic equipment of various types have been miniaturized. Magnetic elements such as inductors and transformers, which are indispensable to the power-supply section of each electronic component can neither be made smaller nor be integrated with the other circuit components, whereas the other circuit sections have successfully been made much smaller in the form of LSIs. Therefore the ratio of the volume of the power-supply section to that of the other sections, combined together, has increased inevitably.
To reduce the sizes of the magnetic elements, such as inductors and transformers, attempts at reduction have been made, and small planar inductors and planar transformers have been achieved. A conventional planar inductor comprises a spiral planar coil, two insulation layers sandwiching the coil, and two magnetic plates sandwiching the coil and insulation layers. A conventional planar transformer comprises two spiral planar coils, used as primary and secondary windings, respectively, two insulation layers sandwiching these coils, and two magnetic layers sandwiching the coils and insulation layers. The spiral planar coils incorporated in the inductor and the transformer can be of either of the two alternative types. The first type is formed of one spiral conductor. The second type comprised of an insulation layer and two spiral conductors mounted on the two major surfaces of the insulation layer, for generating magnetic fields which extend in the same direction.
These planar elements are disclosed in K. Yamasawa et al, High-Frequency of a Planar-Type Microtransformer and Its Application to Multilayered Switching Regulators, IEEE Trans. Mag., Vol. 26, No. 3, May 1990, pp. 1204-1209. As is described in this thesis, the planar elements have a large power loss. Similar planar magnetic elements are disclosed also in U.S. Pat. No. 4,803,609.
It has been proposed that the thin-film process, is employed in order to miniaturize these planar magnetic elements.
Planar inductors of the structure specified above need to have a sufficient quality coefficient Q in the frequency band for which they are used. Planar transformers of the structure described above must have a predetermined gain G which is greater than 1 for raising the input voltage or less than 1 for lowering the input voltage, and must also minimize voltage fluctuation.
The value Q of a planar inductor is:
Q=xcfx89L/R
where R is the resistance of the coil, and L is the inductance of the inductor.
The voltage gain G of a planar transformer without load is:
G=k(L2/L1)1/2{Q/(1+Q2)1/2}
where k is the coupling factor between the primary and secondary windings, L1 and L2 are the inductances of the primary and secondary windings, respectively, the quality coefficient Q is xcfx89L1/R1, and R1 is the resistance of the primary-winding coil. The gain G is virtually proportional to Q when Q less than  less than 1, and has a constant value k(L2/L1)1/2 when Q greater than  greater than 1.
To increase the quality coefficient Q of the inductor, and to increase the gain G of the transformer is thereby to limit the voltage fluctuation, it is necessary to reduce the resistance of, and increase the inductance of, the coil, as much as possible. In the conventional planar magnetic elements made by means of the thin-film process, however, the coil conductors, which need to be formed in a plane, cannot have a large cross-sectional area. Therefore, these elements cannot help but have a very high resistance and an extremely small inductance. Consequently, the conventional planar inductor has an insufficient quality coefficient Q, and the conventional planar transformer has an insufficient gain G and a great voltage fluctuation. These drawbacks of the conventional planar magnetic elements have been a bar to the practical use of these elements.
Of planar coils which can be used in planar inductors, spiral coils are the most preferable due to their great inductance and their great quality coefficient Q. In fact, planar inductors, each having a spiral planar coil, have have been manufactured, one of which is schematically illustrated in FIG. 1. As FIG. 1 shows, the planar inductor comprises a spiral planar coil shaped like a square plate, two polyimide films sandwiching the coil, and two Co-base amorphous alloy ribbons sandwiching the coil and the polyimide films and prepared by cutting a Co-based amorphous alloy foil made by rapidly quenching cooling the melted alloy. This planar inductor is incorporated in an output choke coil for use in a 5 V-2 W DC-DC converter of step-down chopper-type, as is disclosed in N. Sahashi et al, Amorphous Planar Inductor for Small Power Supplies, the National Convention Record, the Institute of Electrical Engineers of Japan 1989, S. 18-5-3. As is evident from the graph of FIG. 2A, two currents flow through this choke coil. The first current is a DC current which corresponds to the load current. The second current is an AC current which has been generated by the operation of a semiconductor switch. As the DC current increases, the operating point of the soft magnetic core, shifts into the saturation region of the B-H curve. As a result, the magnetic permeability of the magnetic alloy lowers, whereby the inductance abruptly decreases as is illustrated in FIG. 2B. As is evident from FIG. 3, the AC current becomes too large at the time the inductance sharply decreases. This excessive AC current is a stress to the semiconductor switch, and may break down the switch in some cases.
It is desired that the choke coil have its electric characteristics, such as inductance, unchanged even if a superimposed DC current flows through it. FIG. 4 is a graph representing the typical superimposed DC current characteristic of the choke coil, which is the relationship between the inductance of an inductor and a superimposed DC current flowing through the inductor.
In the case of a planar inductor, the conductor coil is very close to the soft magnetic cores and, hence, generates an intense magnetic field even if the current flowing through it is rather small. Thus, the soft magnetic cores are likely to undergo magnetic saturation. It will be explained how such magnetic saturation occurs in, for example, a planar inductor which comprises an Alxe2x80x94Cu alloy spiral planar coil, two insulation layers sandwiching the coil, and two magnetic layers clamping the coil and the insulation layers together.
The planar coil of this planar inductor is made of an conductor having a width of 50 xcexcm and a thickness of 10 xcexcm. The coil has 20 turns, and the gap between any two adjacent turns is 10 xcexcm. Each insulation layer has a thickness of 1 xcexcm, and either magnetic layer has a thickness of 5 xcexcm. The planar coil has a saturated magnetic flux density BS of 15 kG and a magnetic permeability xcexcs of 5000.
Assuming that the Alxe2x80x94Cu alloy conductor has a permissible current density of 5xc3x97108 A/m2, the permissible current Imax is 250 mA. The present inventors tested the planar inductor in order to determine the relationship between the current flowing through the coil and the intensity of the magnetic field generated in the surface of either magnetic layer from the current. The results of the test revealed that both magnetic layers were magnetically saturated when a current of 48 mA or more flowed through the Alxe2x80x94Cu alloy coil. It follows that, if this planar inductor is used as a choke coil, the maximum DC superimposed current is limited to 48 mA. This value is no more than about one fifth of the permissible coil current Imax. Inevitably, the magnetic layers will be readily saturated magnetically.
The limited DC superimposed current is a drawback which is serious, not only in the planar inductor used as a choke coil, but also in a planar transformer. In a planar transformer incorporated in, for example, a DC-DC converter of forward type or fly-back type, a pulse voltage of one polarity is applied to the primary coil. The magnetic layers are thereby saturated magnetically, abruptly decreasing the inductance of the transformer.
Hence, attempts have been made to provide a planar inductor and a planar transformer, which are designed such that the influence of the saturation of the magnetic layers is reduced, thereby to increase the maximum DC superimposed current of the device comprising the planar or transformer and to make an effective use of the magnetic anisotropy of the magnetic layers.
Planar coils can be into various types such as zig-zag type, spiral type, zig-zag/spiral type, and so on, in accordance with their patterns. Of these types, the spiral type can be provided with the greatest inductance. Hence, a spiral planar coil can be smaller than any other type having the same inductance. To form the terminals of a spiral planar coil, however, it is necessary to connect two spiral coils positioned in different planes by means of a through-hole conductor, or to use conductors for leading the terminals outwards. Hence, the process of manufacturing a spiral planar coil is more complex than those of manufacturing the other types of planar coils.
For electronic circuit designers it is desirable that planar magnetic elements to be incorporated in an electronic circuit have so-called xe2x80x9ctrimming functionxe2x80x9d so that their characteristics may be adjusted to values suitable for the electronic circuit. A magnetic element having a trimming function has indeed been developed, which has a screw and in which, as the screw is rotated, its position with respect of the core of the coil, thereby to vary the inductance of the magnetic element continuously. However, most conventional planar magnetic elements have no trimming function, for the following reason.
As is known in the art, the characteristics of planar magnetic elements greatly depend on their structural parameters and the characteristics of the planar coils and magnetic layers. These factors determining the characteristics of the magnetic elements depend on the steps of manufacturing the elements. Since these steps can hardly be performed under the same conditions, the resultant elements differ very much in their characteristics. Naturally it is desired that the elements be provided with trimming function. However, they cannot have trimming function because of their specific structural restriction.
Transformer with large output power is disclosed in A. F. Goldberg et al., Issues Related to 1-10-MHz Transformer Design, IEEE Trans. Power Electronics, Vol. 4, No. 1, January 1989, pp. 113-123.
As has been pointed out, planar magnetic elements small enough to be integrated with other circuit elements have not been produced, making it practically impossible to manufacture sufficiently small integrated LC-circuit sections, a typical example of which is a power-supply section.
Since the Multilayered planar inductors essentially have a open magnetic circuit, it is difficult to achieve the following two requirements:
(1) They have no leakage fluxes, and only slightly influence the other components of the IC in which they are in corporated.
(2) They have a large inductance.
Therefore, the multilayered planar inductors cannot serve to provide sufficiently small integrated LC-circuit sections, such as a power-supply section.
Hence, there is still great demand for planar magnetic elements for use in a circuit section, which only slightly influence the other components of the circuit, influence other components. Further, the conventional planar magnetic elements can hardly have trimming function, due to the structural restriction imposed on them.
It is a first object of the present invention to provide a planar magnetic element which is small enough to be integrated with electric elements of other types;
It is a second object of the invention to provide a planar magnetic element which has a sufficiently great inductance;
It is a third object of this invention to provide a planar magnetic element which has but only a few leakage fluxes;
It is a fourth object of the invention to provide a planar magnetic element which excels in high-frequency characteristic and superimposed DC current characteristic;
It is a fifth object of the present invention to provide a planar magnetic element which has large current capacity and, hence, great inductance;
It is a sixth object of the invention to provide a planar magnetic element wherein it is easy to lead terminals outwards;
It is a seventh object of this invention to provide a planar magnetic element which has a trimming function, so that its electric characteristics can be adjusted externally.
The invention will accomplish the above objects by the following six aspects of the invention. According to the invention, the elements of different aspects, each having better characteristics than the conventional ones, can be used in any possible combination, thereby to provide new types of planar elements which have still better characteristics and which have better operability.
According to a first aspect of this invention, there is provided a planar magnetic element which comprises: a spiral planar coil having a gap aspect ratio (i.e., the ratio of the width of the conductor to the gap among the conductors) of at least 1; insulation members laminated with the spiral planar coil; and magnetic members laminated with the insulation members. The coil of this planar magnetic element has a relatively low resistance. Therefore, it will have a large quality coefficient Q when used as an inductor, and will have a great gain when used as a transformer. In other words, the element has a sufficient operability.
According to a second aspect of the present invention, there is provided a planar magnetic element which comprises a planar coil formed of a conductor which has a conductor aspect ratio (i.e., the ratio of the width of the conductor to the thickness thereof) of at least 1. In this regard, it should be noted that when this element is used as an inductor, its ability is determined by its permissible current and inductance. The permissible current is, in turn, determined by the cross-sectional area of the conductor. Hence, the permissible current can be increased by making the conductor broader. If the conductor is made broader, however, it will inevitably occupy a greater area in a plane, which runs counter to the demand for miniaturization of the planar magnetic element. On the other hand, the inductance of the planar magnetic element can indeed be increased by bending the conductor more times, thus forming a coil having more turns. The more turns, the larger the area the coil occupies. This also runs counter to the demand for miniaturization. The planar magnetic element according to the invention can have a sufficiently large permissible current since the conductor has an aspect ratio of at least 1.
According to a third aspect of the invention, there is provided a multilayered planar inductor comprising a spiral planar coil and magnetic members sandwiching the planar coil. The magnetic members have a width w greater than the width a0 of the spiral planar coil by a value more than 2xcex1. It should be noted that the value a is [xcexcsgt/2]1/2 where xcexcs is the relative permeability of the magnetic members, t is the thickness of the magnetic members, and g is the distance between the magnetic members. Since w greater than a0+2xcex1, this planar inductor has a great inductance. When w=a0+2xcex1, for example, the inductance is at least 1.8 times greater than in the case where w=a0. The planar inductor not only has a great inductance, but also has small leakage flux. In view of this, this planar inductor is suitable for use in an integrated circuit, and serves to make electronic devices thinner.
According to a fourth aspect of the present invention, there is provided a planar magnetic element comprising a planar coil and magnetic layers sandwiching the coil. The magnetic layers are magnetically anisotropic in a single axis which extends at right angles to the direction of the magnetic field generated by the coil. Owning to the uniaxial magnetic anisotropy of the magnetic layers, the planar magnetic element excels in superimposed DC current characteristic and high-frequency characteristic. It is suitable for use in high-frequency circuits such as DC-DC converters. In addition, it can be made small and integrated with electric elements of other types, thereby to form an integrated circuit.
According to a fifth aspect of this invention, there is provided a planar magnetic element comprising a planar coil and magnetic layers sandwiching the coil. The planar coil consists of a plurality of one-turn planar coils located in the same plane, having different sizes, and each having an outer terminal. This planar magnetic element can be electrically connected to an external circuit with ease, and can be trimmed by an external means to have its electric characteristics adjusted. Hence, this is a very useful magnetic element, finding use in step-up chopper-type DC-DC converters, resonant DC-DC converters, and very thin RF circuits for use in pagers.
According to a sixth aspect of the present invention, there is provided a planar magnetic element comprising a conductive layer and a magnetic layer. The magnetic layer surrounds the conductive layer, thus forming a closed magnetic circuit. The current flowing in the conductor layer magnetizes the magnetic layer in the direction of the closed magnetic circuit. This planar magnetic element has small leakage flux and a great current capacity. It can, therefore, serve to render electronic devices thinner when incorporated into these devices.
The planar magnetic elements of the invention, described above, can not only be small but also have improved characteristics generally required of magnetic elements such as inductors.
The planar inductors and transformers according to the invention, which comprise planar micro-coils, are small and can be formed on a semiconductor substrate. Therefore, they can be integrated with active elements (e.g., transistors) and passive elements (e.g., resistors and capacitors), thereby constituting a one-chip semiconductor device. In other words, they help to provide small-sized electronic devices containing inductors and transformers. In addition, the planar inductors and transformers of the invention can be fabricated by means of the existing micro-technique commonly applied to the manufacture of semiconductor devices.
As can be understood from the above, the present invention serve to provide small and thin LC-circuit sections for use in various electronic devices, and ultimately contributes to the miniaturization of the electronic devices.